1. Field of the Invention
This invention relates to a control circuit for driving a step motor.
2. Description of the Prior Art
Step motors (also called stepping or pulse motors) in general have a rotor which rotates in a predetermined step-degree arc for one step pulse and are widely used in moving portions of automatic machines such as x-y plotters, x-y recorders and industrial robots, because the angle of rotation of the rotor is controllable according to the number of input step pulses.
There is a known control circuit for driving such a step motor to rotate in such a manner as to make memory elements such as programmable ROMs (Read Only Memories) store phase switching data beforehand, count step pulses using address counters to access the data stored in the memory elements according to the outputs of the counters, and control the "on" and "off" states of the current flowing through the motor's different phase coils based on the phase switching data read out.
The phase switching data stored in the memory elements is step switching data for rotating the rotor of the motor step angle by step angle as predetermined according to the step pulse and the data has such contents as shown in Table 1.
TABLE 1 ______________________________________ Address: Phase Switching Data: Step: A.sub.2 A.sub.1 A.sub.0 Phase .sup.--B Phase .sup.--A Phase B Phase A ______________________________________ 0 0 0 0 0 0 0 1 1 0 0 1 0 0 1 1 2 0 1 0 0 0 1 0 3 0 1 1 0 1 1 0 4 1 0 0 0 1 0 0 5 1 0 1 1 1 0 0 6 1 1 0 1 0 0 0 7 1 1 1 1 0 0 1 ______________________________________
Table 1 above shows an example of the phase switching data wherein a four-phase step motor with phases A,B,A,B is one-two phase excited by a unipolar driving circuit to set 8 steps as a repeat period, whereby (1) and (0) correspond to ON and OFF, respectively. Since the motor can thus be driven without changing the circuit through other exciting methods by rewriting the data of Table 1, such phase signal control circuits for producing phase switching signals using memory elements are widely in use.
In the steps 0,2,4,6 of Table 1, an excitation current is allowed to flow through the coil of only one of the phases A,B,A,B (one phase excitation) on the one hand, and the excitation current is caused to flow through the coil of phases A,B simultaneously (two phase excitation) in the other steps, for instance, in step 1. Accordingly, the generated motor torque in one phase excitation and the unbalance between them becomes the source of vibration, noise, or an error in angle when a load is applied. Moreover, the maximum number of steps obtainable using the four phase coil in the unipolar driving method is 8, which is the case with one-two phase excitation drive. Although one step in that case is equivalent to 1/2 of a conventional step (the so-called one full step) in view of the standard terminology of the step motor, it may have to be driven in small steps with high resolution. When the motor speed and motor load differ, it is also desirous that the torque should be selected according to the speed and load.
Heretofore, attempts have been made to stablize the balance of torque, turn the motor by steps smaller than the full step of 1/2 and control the torque in proportion to the motor speed and motor load by adding an analog circuit such as a current control circuit to the digital circuit. However, the disadvantage is that the addition of such an analog circuit not only makes the circuit arrangement more complex and expensive but also creates some difficulty in design alteration.